1. Field of the Invention
The present invention relates to an ion generating apparatus that applies an alternating voltage between an induction electrode and a discharge electrode to cause corona discharge and thereby generates both positive and negative ions. The present invention relates also to an air conditioning apparatus and a charging apparatus provided with such an ion generating apparatus.
2. Description of the Prior Art
There is conventionally known a corona discharge element so structured that a dielectric layer is sandwiched between an induction electrode and a discharge electrode. An example of a so structured corona discharge element is disclosed, for example, in Japanese Patent Application Published No. H2-22998 (hereinafter referred to as Patent Reference 1). This corona discharge element is a surface corona discharge element composed of a 0.5 mm thick piece of alumna porcelain having a line-shaped discharge electrode of tungsten formed on one side thereof and having a surface-shaped induction electrode formed on the other side thereof This type of corona discharge element is used, for example, as an ozonizer.
In the manufacturing process of this corona discharge element, to form the tungsten discharge electrode on the alumina substrate, it is necessary to go through a step of high-temperature baking at 1,500° C. Moreover, to enable the corona discharge element to cause electric discharge, it is necessary to apply a voltage as high as 10 kVpp (peak-to-peak) at 10 kHz between the induction electrode and the discharge electrode. This necessitates special consideration for reliability and safety against human body contact and malfunctioning. Moreover, the high-voltage power supply by itself is not only expensive but highly power-consuming.
This corona discharge element operates with good ozone generation efficiency, and is therefore suitable for use as a ozonizer. It is difficult, however, to use it in an air purifier or charging apparatus because it generates too much ozone, which is hazardous to the human body.
There is also conventionally known an example in which a discharge element structured similarly to the one described above is applied in a charging apparatus. For example, U.S. Pat. No. 4,155,093 (hereinafter referred to as Patent Reference 2) discloses, as an example of such a discharge element, a discharge element composed of a piece of glass having line-shaped electrodes arranged on opposite sides thereof so as to cross each other. In this structure, electric discharge occurs and ions are generated selectively at the intersections between the line-shaped electrodes on one side and those on the opposite side. This makes it possible to form an electrostatic latent image directly on a cylindrical dielectric member placed so as to face the discharge element. By making this electrostatic latent image visible on the principle of electrophotography, it is possible to realize a printer, copier, facsimile machine, or the like.
There have also been conventionally made many proposals to use a discharge element not as a charging apparatus as described above but as a charger that discharges uniformly in the axial direction of the discharge element to charge a photoconductive member for electrophotography. Also in such applications in a charging apparatus or charger, however, as described above, it is necessary to go through a step of high-temperature baking in the manufacturing process, to use a high-voltage power supply, and to use an ozone-eliminating filter because of the large amount of ozone that the discharge element generates.
There have also been conventionally proposed discharge elements of a different type from the one described above. For example, Japanese Patent Application Laid-Open No. 2002-95731 (hereinafter referred to as Patent Reference 3) discloses a discharge element that uses a cylindrical glass tube as a dielectric layer and that is applied in an air conditioning apparatus so that positive ions H+(H2O)m (where m is a natural number) and negative ions O2−(H2O)n (where n is a natural number) are generated by electric discharge and they are used to kill airborne bacteria floating in the atmosphere.
Also in this type of discharge element, since ions are generated on the principle of electric discharge, ozone is inevitably generated together. Since ozone is hazardous to the human body, its permissible concentration, i.e., safe level, is regulated as 0.1 ppm by Japan Society for Occupational Health. Accordingly, in the air conditioning apparatus mentioned above, to limit the amount of ozone generated below that safe level, there is provided an ozone concentration detecting sensor so that, according to the ozone concentration detected, a controller controls the voltage applied to the discharge element and other parameters. Here, the air conditioning apparatus requires the additional provision of the ozone concentration detecting sensor and the controller, and this increases the costs and size of the air conditioning apparatus.
Incidentally, as dealt with in an article included in “Journal of Imaging Science,” Vol. 32, No. 5, pp. 205–210, September/October 1988 (hereinafter referred to as Non-Patent Reference 1), research has been being done on the relationship between the wire diameter of a wire electrode to which a high voltage is applied to cause corona discharge and the amount of ozone generated. This article shows that, in experiments conducted with wire electrodes of diameters of several ten μm to 150 μm, there is a linear relationship such that, the smaller the wire diameter, the smaller the amount of ozone generated. It is also shown that this tendency is observed similarly both in positive and negative corona but that the amount of ozone generated by positive corona is smaller by about one order of magnitude than that generated by negative corona. These discharge characteristics are the results of studying the characteristics of a discharge element used as the discharger of a copier, and therefore they are considered to suggest that the same quantity of ions for electric discharge can be generated with a reduced amount of ozone, which is hazardous to the human body.
However, in the structure disclosed in Patent Document 1 and described above, the dielectric layer sandwiched between the discharge electrode and the induction electrode is a ceramic substrate, such as one made of alumina porcelain. Since ceramic is a brittle material, inconveniently, the larger the size of the discharge element, the lower the mechanical rupture strength thereof
Moreover, the conventional discharge element (ion generating apparatus) is so structured as to generate both positive and negative ions by applying a high voltage between the discharge electrode and the induction electrode. Here, the application of the high voltage results in generating a large amount of ozone, which is hazardous to the human body, and therefore using such a discharge element in an air conditioning apparatus or charging apparatus is suspected of leading to a health hazard.